<p>Probabilistic planning is an evolving approach for tumor motion management in which reproducibility of probability distribution function (PDF) of tumor motion is critical yet unclear. Aim of the first study is to evaluate the reproducibility of tumor motion PDF in stereotactic body radiation therapy (SBRT) using cine megavoltage (MV) images. External surrogate is used clinically in 4DCT imaging and radiation treatments for respiratory motion monitoring. However, studies have shown questionable correlation between external surrogate motion and internal tumor motion. Thus, Aim of the second study is to evaluate the correlation of external surrogate motion and internal tumor motion from a statistical point of view. </p><p>20 lung cancer patients who underwent SBRT treatment using 3D conformal technique were included in our study. During simulation, 4DCT scan assisted with RPM system was done. Cine MV images acquired during treatments were collected to extract tumor motion trajectories. For each patient, tumor motion PDFn was generated using 3 "usable" beams for each fraction. Patients without at least 3 "usable" beams were excluded. PDFn reproducibility (Rn) was calculated using the Dice Coefficient between PDFn to a "ground-truth" PDF (PDFg). The mean of Rn (Rm) was calculated for each patient and correlated to mean tumor motion rang (Am). Change of Rm during the course of SBRT treatments was also evaluated. </p><p>Thirteen patients were kept for further analysis. The tumor motion PDF during the treatments can be determined using cine MV images. The reproducibility of lung tumor motion PDF decreased exponentially as the tumor motion range increased and also decreased slightly throughout the course of treatments. </p><p>For each of thirteen patients, tumor motion range, tumor motion "ground-truth" PDFg (PDF<sub>MV</sub>), and tumor motion variability VMV were calculated using the cine-MV images. Similarly, surrogate motion range (R<sub>MV</sub>), surrogate PDF (PDF<sub>RPM</sub>), and surrogate variability (V<sub>RPM</sub>) were calculated using motion trajectory of the reflective marker. Correlation between and RRPM, and between VMV and VRPM, and between similarity of PDF<sub>MV</sub> and PDF<sub>RPM</sub> and R<sub>MV were determined.</p><p>No correlations were found in motion range and variability between the external surrogate (RPM) and the internal lung tumor motion. High PDF similarity, with a mean (±standard deviation) of 0.83(±0.1) was found between RPM and internal lung tumor motion, but no correlation exists between this PDF similarity and tumor motion range.</p>